Our long-term goal is to investigate the mechanisms responsible for graft-versus-host disease (GVHD) pathogenesis, which will lead to new approaches designed to inhibit GVHD. Emerging data have indicated that the presence of pro- or anti-inflammatory signals in the local environment control naove T cell differentiation into pathogenic or suppressor cells, respectively, which have a reciprocal relationship. Recent data indicate that there is a cellular regulatory network operating at the tissue level to control the decision and fate of naove T cells. The network is created by the interaction of T cell immunoglobulin, mucin-containing type I membrane glycoproteins (TIM family) present on T cells and S-type lectins (galectins) that recognize TIM carbohydrates present on immune system cells and on a wide range of tissue cells. Tim-3 is expressed on differentiated T effector cells with the highest density on pathogenic Th1 and Th17 cells. Galectin-9 (gal-9) has been identified the ligand for Tim-3, and is upregulated in inflamed tissues. A major function of the Tim-3/gal-9 is to limit immune responses under conditions of tissue inflammation and injury. Conversely, in vivo blockade of Tim- 3/gal-9 interaction increases Th1 cells within inflamed tissues. In contrast to the inhibitory Tim-3 signals on T effectors, gal-9 signals appear to support CD4+25+ regulatory T cell (Treg) generation and function. Our preliminary data indicate that the Tim-3/gal-9 pathway is upregulated in GVHD tissues and on T effector cells. We have found that the Tim-3/gal-9 pathway is a major regulator of GVHD lethality. Surprisingly dichotomous effects of pathway blockade were seen in recipients of unmanipulated vs. Treg-depleted T cell grafts. Two aims are proposed that will provide insights into GVHD pathogenesis, Treg cell biology, and further elucidate the unique mechanisms by which the Tim-3/gal-9 pathway regulates immune responses. In aim 1, we will define the Tim-3/gal-9 dependent mechanisms regulating GVHD pathogenesis and severity. In aim 2, we will devise novel Tim-3/gal-9 based clinically relevant therapeutic approaches to inhibit GVHD early and to augment graft- versus-leukemia later post-BMT. PUBLIC HEALTH RELEVANCE: Our goal is to develop clinically relevant approaches that will facilitate adoptive T cell immunotherapy to treat patients with cancer. The fundamental insights gained from these studies will have broad implications relevant to both cancer therapy and treatment of infectious diseases.